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IV capture with Wireshark
Like moshbat said earlier, you need to be connected to the network in order to be able to sniff packets in the networks, and since you mentioned sending them to aircrack, that means you want to crack the wep password first before connecting to the network right?
Once you're already in the network, however, you could decrypt WEP-encrypted data.
So I would say that firstly, you have to crack it with Aircrack, get the key, and then you would be able to sniff the packets coming around the network.
Here's some guides:
http://www.twistedethics.com/2007/05/04/how-to-crack-wep-in-windows-with-aircrack-ng-and-airpcap/
Best bet: http://wirelessdefence.org/Contents/Aircrack_airodump.htm
Works great for me.